After Mariner 10, it took until 2004 for another spacecraft to orbit Mercury: the MESSENGER mission (MErcury Surface, Space ENvironment, GEochemistry and Ranging). MESSENGER successfully completed an exhaustive map of Mercury’s surface as well as confirmed water ice in polar craters.
Europe and Japan’s joint BepiColombo probe is gearing up for its seven-year journey to explore Mercury’s cratered surface and mysterious magnetic properties. Its design includes two orbiters – one European and one Japanese – which will separate upon arriving.
Mariner 10
Mariner 10 was the seventh spacecraft to reach Mercury and was notable for several firsts: it used Venus’ gravitational assist maneuver to enter heliocentric orbit around Mercury; created a full color surface map; and featured an experimental “X-band” high frequency transmitter.
Mariner 10 conducted three flybys of Mercury that greatly advanced scientific understanding. For instance, its findings include that over one third of its surface is covered with craters similar to Earth’s Moon; also, an atmospheric layer and magnetic field were discovered by Mariner 10.
Mariner 10 entered a long orbit that allowed it to visit Mercury every 176 days, which proved beneficial for its successor mission, Messenger (MErcury Surface, Space ENvironment, Geochemistry and Ranging), which confirmed water ice at Mercury’s north pole in 2011. Messenger continued orbiting until running out of fuel and colliding into it in April 2015 before crashing on its surface – suggesting comets or meteorites might have delivered the ice directly.
MESSENGER
Over four years, MESSENGER orbited Mercury in an elliptical orbit and measured its surface, gravity field and magnetosphere – providing unprecedented insights into this planet’s characteristics and returning vast amounts of data that revolutionized our understanding of it.
Scientists also discovered that Mercury’s polar regions contain relatively recent ice accumulation, possibly just over 10 million years old. Furthermore, their magnetic field differs significantly from Earth’s and is offset by 10% of its radius.
At the tail-end of its mission, MESSENGER ran out of fuel; but thanks to clever use of cold helium gas for pressurizing rocket propellant in its thrusters, its mission could be extended twice further.
On April 30, 2015, its final pass was inevitable. On that date, MESSENGER crashed into Mercury’s surface at high velocity – something scientists had anticipated happening – with its wreckage becoming part of Janacek crater. Although this collision was unplanned by anyone involved, its legacy lives on through spectacular pictures and useful science discoveries made during its mission.
BepiColombo
BepiColombo, a joint mission between European Space Agency (ESA) and Japan Aerospace Exploration Agency (JAXA), was officially launched on October 20, 2018 under Giuseppe (Bepi) Colombo – an Italian mathematician who used Venus gravity-assist maneuver to allow Mariner 10 spacecraft to pass Mercury three times between 1974-5 – and U-M Climate & Space scientists played key roles on both science orbiters’ teams: Mercury Planetary Orbiter (MPO) and Mercury Magnetospheric Orbiter (MMO). U-M Climate & Space scientists played key roles on both Mercury Planetary Orbiter (MPO) and Mercury Magnetospheric Orbiter (MMO).
BepiColombo will build on the discoveries made by MESSENGER and Mariner 10 by providing a global characterization of Mercury by exploring its interior, surface, exosphere, magnetosphere and magnetotail regions. Furthermore, instrumentation within BepiColombo will test Einstein’s theory of general relativity.
BepiColombo comprises two satellites designed by ESA: Mercury Planetary Orbiter with 11 instruments dedicated to studying its surface, internal structure, composition and atmosphere; and Mio, built by JAXA for studying Mercury’s magnetic field and surrounding space environment. Both satellites will be carried to Mercury by an MTM equipped with solar electric ion thrusters which facilitate interplanetary travel.
Cassini
Cassini completed its mission by self-destructing in Saturn’s atmosphere, leaving behind an abundance of information for scientists to sift through. NASA deliberately killed off Cassini to prevent it from accidentally hitting two moons — Enceladus with its subsurface ocean and Titan, believed by some scientists to harbor signs of extraterrestrial life – during its mission.
Arecibo and other telescopes have made amazing discoveries about Mercury, but sunlight makes studying it challenging in daylight hours. Even the iconic Hubble Space Telescope has never observed Mercury due to the risk it would pose to its delicate optics.
For this mission, MESSENGER used two swingbys of Venus separated by a year to adjust its orbit until it resonated with Mercury’s 176-day period. Because its orbit is elliptical – with its periapsis closer to Mercury than its apoapsis – multiple mapping sessions could take place during its mission, giving scientists access to this innermost planet’s surface and gravitational field data multiple times throughout.